A desire to increase both the data rate and transmission distance of optical communication links has prompted the use of coherent signaling. Conventionally, optical communication systems have relied on the use of simple signaling methods to encode data bits onto an optical carrier. The most common signaling method is intensity modulation, in which a laser is gated to allow high intensity light to enter a fiber optical cable when a ‘1’ bit is to be transmitted and low intensity light when a ‘0’ bit is transmitted. This is a form of on-off keying and has the advantage that it is easily demodulated by a photo-diode and an appropriate threshold.
The main drawback of on-off keying is that the bandwidth efficiency is low, due to the fact that information is only transmitted in a single dimension. Coherent signaling techniques, on the other hand, allow for the transmission of multidimensional signals, by modulating both the intensity and the phase of the laser light.
FIG. 1 shows a prior art coherent optical communication system. A transmitter 100 includes a first laser light source (transmit Local Oscilator—TXLO), 101, whose output is a constant beam of light, or pulse trains if pulse carvers are used. The beam is input to a modulator 102, which modulates both the amplitude and the phase of the input light beam according to a data source 103. The data source in the transmitter typically includes a forward error correction (FEC) encoder. Thus, the combination of laser and modulator is capable of generating any common two dimensional digital modulation format, e.g., phase shift keying (PSK) and quadrature amplitude modulation (QAM).
The dimensional signal is passed through an optical channel, 104, and is detected and demodulated in a receiver 105. A coherent receiver includes a second laser light source (receive Local Oscilator RXLO) 101′, an optical hybrid demodulator and photo detectors (termed a “coherent detector”), 106, and an electrical digital receiver 107.
Several impairments affect the performance of the optical communication link, including the effect of non-ideal lasers. An ideal laser output can be expressed asEcw(t)=√{square root over (Ps)} exp(jωst+θs)es,where Ecw(t) is an optical carrier, t is time √{square root over (Ps)} is the an amplitude, ωs is a frequency in radians per second, θs is an initial phase, and es is a polarization of the optical carrier. Deviations from the above ideal laser output are caused by spontaneous emitted photons, which cause intensity and phase fluctuations and result in a laser output that can be expressed asEcw(t)=√{square root over (Ps+δP(t))}exp(jωst+θs+θns(t))es,where δP(t) and θns(t) represent intensity and phase noise processes, respectively.
Because these processes are due to the spontaneous emissions, they are reasonably modeled as Gaussian distributed random processes. The effect of the phase noise is to broaden a power spectral density of the optical field. Thus, the laser output is no longer confined to a single frequency tone, but typically has a Lorentzian-shaped spectrum. The full-width half maximum (FWHM) bandwidth is normally termed the “laser linewidth.” The phase noise, θns(t), or equivalently the laser linewidth, is particularly troublesome for coherent systems in that the phase of the carrier Ecw(t) is needed to coherently demodulate any two dimensional modulation format.
The random carrier phase θns(t) impairs carrier recovery processes, and generally the larger the laser linewidth, the more difficult it is for the receiver to track the carrier phase changes. The coherent receiver makes use of an additional laser 101′ to generate a local version of the optical carrier for mixing with the received signal. The non-ideal linewidth of this laser also compound the effects of the transmit laser's linewidth. Typical linewidth requirements for lasers in coherent systems are on the order of tens to hundreds of kHz, whereas direct detection or differentially Quadrature Phase Shift Keying (QPSK) can greatly reduce the linewidth requirements to tens of MHz because the signal formats are less sensitive to carrier phase changes. Studies show that a QPSK system requires maximum linewidth of 250 KHz, and a 16QAM systems requires 6.9 KHz linewidth.
In addition to laser linewidth, the received signal also experiences linear dispersive effects such as chromatic dispersion, as well as nonlinear effects, such as fourwave mixing as the signal traverses the optical fiber. Typical receivers make use of electronic signal processing to reduce the dispersive and non-linear effects. Thus, it is desirable to compensate for laser linewidth effects using electronic signal processing techniques as well.
In addition to the zero-mean phase noise, carrier frequency offset (CFO) ΔF may exist between two oscillators due to the temperature variation, aging and other slow effects. CFO is generally very slow with regard to the symbol rate, and therefore can be considered constant over a number of transmitted symbols. CFO adds additional phase error.
The non-zero linewidth Δv of the local oscillator (LO) results in a phase noise from one received symbol r(n) to the next symbol r(n+1) is represented byδθk=θk−θk−1=Ts[(0,(2πΔv)2)+ΔF], where Ts is the symbol duration and (0, σ2) denotes a zero mean normal distribution with variance, σ2. The method described in this invention, however, does not require the phase noise distribution to be normal, or zero-mean.
The phase error of the kth symbol can be expressed as
                              θ          k                =                                                            ∑                                  i                  =                  1                                k                            ⁢                              δθ                i                                      +                          θ              0                                =                                    θ              0                        +                                          T                s                            ⁢                                                ∑                                      i                    =                    1                                    k                                ⁢                                  N                  ⁡                                      (                                          0                      ,                                              2                        ⁢                        πΔν                                                              )                                                                        +                                          kT                S                            ⁢              Δ              ⁢                                                          ⁢              F                                                              =                              θ            m                    +                                    T              s                        ⁢                                          ∑                                  i                  =                                      m                    +                    1                                                  k                            ⁢                              N                ⁡                                  (                                      0                    ,                                          2                      ⁢                      πΔν                                                        )                                                              +                                    (                              k                -                m                            )                        ⁢                          T              S                        ⁢            Δ            ⁢                                                  ⁢                          F              .                                          
FIG. 2A shows a zero-mean distribution of the phase noise when the laser linewidth is 5 MHz and the symbol rate is 25 G Hz. The distribution approximates a normal distribution, and the majority of the phase noise is within +/−6×10−3 radian.
FIG. 2B shows the phase error over 100,000 symbols, with the phase of the initial symbol is 0 rad. In the example, the accumulated phase error is 0.8 rad. Clearly, a coherent system with such high LO linewidth cannot function properly without proper phase compensation. FIG. 2B also shows that the phase error is increased in the presence of CFO. In the example, ΔF=50 kHz.
To limit the phase error, most conventional systems use LOs with very narrow linewidth. Because the requirement of the linewidth is also related to the modulation scheme, modulations with higher spectrum efficiency requires a much tighter LO linewidth.
To achieve the required performance, conventional coherent optical systems use lasers with very narrow linewidth as local oscillators, typically external cavity distributed feedback (DFB) lasers. These lasers have the linewidth Δv in the tens of kHz range, but are very expensive, have larger form factor, and higher power consumption.
Many other laser sources such as DFB lasers and vertical-cavity surface-emitting laser (VCSELs) are more cost and power efficient. However, the strict linewidth requirement prohibits the use of these inexpensive, and more energy efficient lasers in coherent systems, as the receiver performance degrades rapidly with increasing linewidth. Most of the DFB lasers, VCSELs and tunable lasers have linewidth that is in the range of MHz. Conventional current coherent systems will not function with adequate performance under such large linewidth.
Therefore, it is desirable to design coherent systems with relaxed linewidth tolerance to allow lower cost, or tunable light sources to be used in the coherent system. This will significantly lower the overall system cost and energy consumption. Tunable lasers also provide flexibility and reconfigurability to coherent systems.